The invention relates to a directional stability controller for a motor vehicle. U.S. Pat. Nos. 5,029,090 and 4,794,539 describe controllers of the type which use a single-track model to determine a desired value of the yaw velocity or a desired value of the difference in speeds of the steered wheels from the steering angle and the vehicle speed. The deviation between the desired value and the actual value is then used to control engine torque.
In the case of vehicle controllers of this kind, which are intended to control the transverse dynamics of the vehicle, the actual vehicle movement must be compared with the intended movement. For this purpose, the driver's intention is determined by measuring the steering angle. The intended vehicle movement (yaw velocity) is then determined by means of a linear single-track model using the measured vehicle speed. The linear single-track model comprises two differential equations: ##EQU1## in which: .psi..sub.s =intended yaw velocity
.beta.=attitude speed PA1 .nu.=vehicle speed PA1 .delta..sub.h =wheel steer angle, rear PA1 .delta..sub.r =wheel steer angle, front PA1 .delta.=steering-wheel angle PA1 .theta..sub.z =moment of inertia about the vertical axis PA1 l.sub.v =centroidal distance, front PA1 l.sub.h =centroidal distance, rear PA1 m.sub.g =total mass PA1 c.sub.v =slip stiffness, front PA1 c.sub.h =slip stiffness, rear PA1 s=j.omega. PA1 in1=in PA1 where a1=-(1-dt*c)/(1+dt*c)
This gives the (yaw) transfer function: ##EQU2## where .delta..sub.h =f.sub.h *.delta..sub.r, and f.sub.h is the relationship between the front-axle wheel steer angle .delta..sub.r and the rear-axle steer angle .delta..sub.h. Without rear-axle steering, .delta..sub.h =0.
As an alternative to the yaw velocity .psi..sub.s, it is also possible to use the difference between the speeds of the wheels of the undriven axle, the actual value being measured and the desired value being determined by means of the single-track model.
For high frequencies (in the vicinity of the characteristic frequency), the frequency response of the yaw transfer function of the linear single-track model has a phase shift of -.pi./2. Actual vehicles have a phase shift of about -.pi. in this range (due to additional elasticities in the running gear and to the tire recovery length).
Since the phase shift of the linear single-track model differs from that of the actual vehicle, dynamic maneuvers (rapid lane changes, turn-off procedures) lead to a phase error in the calculation of the desired value. The desired value arrives at a figure before the actual vehicle value has time to reach it. As a result, a deviation between the desired and the actual movement is erroneously observed and an unwarranted control operation (e.g. reduction in engine power) is initiated.
In order to compensate for this difference, the desired values of the last 200 ms have, in an internal development, hitherto been stored in a ring buffer store. To determine the system deviation, the ring buffer store was searched for the desired value which differed the least from the actual value. This difference was then used as the system deviation. Under certain circumstances, this procedure made the system deviation too small and, as a result, the phase compensation in these cases inevitably hindered. This led to jumps in the characteristic of the system deviation.